1. Field of the Invention
The invention relates to gas turbine apparatus for the generation of electrical power on a domestic scale, preferably with the simultaneous production of heat usable for domestic heating.
The simultaneous and in situ generation of heat and electric power (known as cogeneration) on a domestic scale can be advantageous in that a greater total energy efficiency for a particular building and for the nation is possible than when the electrical power is generated centrally for distribution to the building.
Legislation in most countries now permits and encourages private generation of electrical power especially for combined heat and power projects. This legislation also allows private generators to operate electrical generating equipment in parallel with the main supply.
Currently the smallest commercial cogeneration equipment available has an electrical power rating of approximately 15 kW. In general, packaged cogeneration equipment of this size is too large for domestic use where electrical power ratings of approximately 1 kW would be more appropriate.
2. Description of the Related Art
To date, low power cogeneration systems (PCSs) have almost exclusively been based on reciprocating engines of the automotive type and converted in most cases to run on fuels such as natural gas. The manufacturer of low power PCSs has benefitted from the economies of scale associated with automotive production. PCSs of this type, such as that of U.S. Pat. No. 4,226,214, yield total energy efficiencies approaching 90% and can theoretically achieve payback periods of less than three years.
However, in small scale applications such as a domestic combined heat and power unit, the reciprocating engine suffers from at least the following drawbacks; noise and vibration, the need for regular maintenance and lastly, a relatively short operating life.
Gas turbine based cogeneration systems currently tend to have larger power outputs, typically at least about one megawatt, and very few companies offer a PCS based on a gas turbine which has a low power rating.
Gas turbine PCSs for domestic use are for the present not available because in order to achieve low shaft output powers, conventional gas turbines either must be very small and thus inherently be very high speed devices or must operate under conditions which are not optimum from a design point of view, i.e. on part load. Each of these methods is inherently disadvantageous.
In the first instance, the low power implies a small air mass flow rate which dictates the need for miniature components which operate at high speed as a direct result of increasing pressure ratio and density. Such small components theoretically have high efficiencies but in practice constructional tolerances and operating clearances diminish the thermodynamic performance. Part load operation of gas turbines in general requires a reduced turbine entry temperature and lower pressure ratios which, by the laws of thermodynamics, results in a lower brake power thermal efficiency.
Mechanical losses will account for proportionally more of the shaft output power as a larger machine is operated at part load, reducing the brake power thermal efficiencies still further.
A further disadvantage of conventional gas turbines is the high combustion pressure which necessitates a higher fuel pressure. The requirement to pressurize the fuel results in the output shaft power being negated by the amount of work necessary to pressurize the fuel, further decreasing the brake power thermal efficiencies. Also, provision must be made in the plant for pressurizing the fuel which necessitates additional space requirements and expense. However, in the domestic situation a gas turbine based PCS has the advantages of reduced maintenance, greater design life and reduced noise and vibration, making it compatible with the intended environment.